476.—10- or 12-inch Theodolite.—These instruments approach the limit in size of portability for movable stations in triangulation. The illustration, Fig. 196, is of the author's latest 10-inch model, the patterns of which were made for a theodolite with vernier readings to be used for the construction of a spiral tunnel through the Andes, now completed. The object in its construction was to obtain great rigidity with moderate weight. To this end the gun-metal of which it is made is shaped out from castings as comprehensive in unity of parts as possible. It has a framed mahogany stand (not shown) which is braced in every way and provided with a very rigid head. In general construction of the instrument illustrated it has a mechanical sliding stage and an extra powerful clamp and tangent arrangement for the lower limb, the adjusting screws being all covered to exclude dust. The circles are divided to 5 minutes, and are read by micrometers to single seconds of arc. Two vertical arcs are used, the second one carrying the clamp and tangent arrangements, which also serves to balance the trunnion. The 10-inch instrument carries a 16-inch telescope with 2-inch object-glass, and the 12-inch instrument an 18-inch telescope with 2-1/8-inch object-glass. The tangent screws all act against springs to avoid loss of time.
Fig. 197.—14-inch altazimuth theodolite.
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477.—14-inch Theodolite.—For this description a modern instrument is taken which Colonel A. R. Clark selected for illustration in his excellent article on Geodesy in the ninth edition of the Encyclopædia Britannica, to the publishers of which the author is indebted for the illustration, Fig. 197. The instrument is a combination of a transit theodolite with special arrangements as an altazimuth instrument with fixed base, one side of the vertical circle being divided to place the zero in a direction coincident with the polar axis. The construction as a simple fixed transit theodolite for support upon a pedestal in stone wherein the axis remains permanent for a principal geodetic station, and therefore requires only a single setting to bring it to true north and south for zero, renders change of position of horizontal limb unnecessary for a permanent station. For this the instrument is well adapted, and will be discussed here. The telescope is of 18 inches focus, with 2 inches clear object-glass. The axis pivots are of hard steel: one is perforated for illumination by a lamp. The vertical circle is placed almost directly upon the side of the telescope, and the tangent arm on the opposite side is of nearly equal weight, so that there is no counterbalance necessary. There are three Ramsden eye-pieces giving powers of 17, 35, and 54, and one diagonal eye-piece. A level is attached inside the standard, divided to read 10″ of arc: this has cemented ends, art. 177, and is enclosed in an outer tube for protection. Two other exactly similar levels are attached to the exterior axis of the instrument. The circle is divided to 5′ of arc and reads by two micrometer microscopes to single seconds. The vertical axis of the instrument is of steel. It is placed with the apex of the cone upwards, and terminates on a triangular spring with three adjusting screws by which any portion of the weight of the upper part of the instrument can be relieved from the axis, so that the whole instrument moves quite freely. The horizontal circle reads with three micrometer microscopes on the upper circle to single seconds. Originally the light was thrown down on the divisions by three ivory cones placed over the fronts of the microscopes, as shown in the illustration; but these have been changed in the present instrument for concave swivelled reflectors, which may be set to any angle convenient to throw sufficient light upon the circle. The microscopes are supported from the body of the instrument upon hollow conical arms upon the same excellent plan originally used by Ramsden. The microscopes have adjustments in three directions, so as to bring them exactly into place for trisection of the horizontal circle. The clamp and tangent motion is placed directly upon the divided circle, and has adjustments to secure freedom from strain; but this is not perfect—it is perhaps the worst feature in the instrument, some modification of the plan shown, [Fig. 196], being much better for large instruments.
The whole instrument is mounted on a tribrach frame, which is adapted to stand upon a portable table or upon masonry. The screws have lateral adjustment to prevent loss of time by wear.
478.—It is a common custom with this class of instrument to make the axes of hard steel. This plan is no doubt very satisfactory as it leaves the optician's hands, but the author very much prefers good hard bell-metal. When he saw the above described instrument at Southampton, there was quite sufficient evidence of rust on the pivots to destroy all perfection of centring, and this could scarcely have occurred with bell-metal. Of course the brittleness of bell-metal would be objectionable where the instrument might be subjected to severe jar in carriage from place to place; but the author has obviated this by a plan he would strongly recommend for general adoption—of having the axis of good gun-metal, and to silver-solder a ring of bell-metal thereon where the fitting surfaces occur. If the gun-metal is pure it will bear the average reliable strain of hardened steel, which in hardening and tempering is not with certainty always free from flaws; and the average wear of pure bell-metal is perhaps quite as good as steel.
479.—36-inch Theodolite, Fig. 198, was designed by the late Colonel A. Strange and constructed by Messrs. Troughton & Simms for the Great Trigonometrical Survey of India. It is probably the most complete and perfect theodolite ever constructed. The leading characteristics of this important instrument only will be given. It has a horizontal circle 36 inches diameter, and a vertical circle 24 inches diameter. The telescope has a focal length of 36 inches: the aperture of the object-glass is 3·25 inches.
Fig. 198.—36-inch theodolite—Great Indian Survey. From a photograph.